Piezoelectric pressure and temperature transducers have been known for some time. Such transducers typically comprise a crystal resonator located inside a housing structure. Electrodes are placed on opposite sides of the resonator to provide a vibration-exciting field in the resonator. Environmental pressure and temperature are transmitted to the resonator via the housing and the stresses in the resonator alter the vibrational characteristics of the resonator, this alteration being sensed and used to interpret the pressure and/or temperature. U.S. Pat. No. 3,617,780 describes on form of such a transducer which comprises a unitary piezoelectric crystal resonator and housing structure in which the resonator is positioned on a median (radial) plane of the cylindrical housing. Crystal end caps are located at either end of the housing to complete the structure of the transducer. Since the vibration of the resonator is affected by both temperature and pressure, such devices can be difficult to use in environments where both vary in an uncontrolled manner. Such devices are known as single mode transducers.
One proposal to overcome this drawback of single mode transducers is described in U.S. Pat. No. 5,471,882. In this case and instrument is provided with two single mode transducers configured to have different temperature responses but similar pressure responses. By comparing the output of the two, the temperature effect can be cancelled. Another approach is to isolate one of the transducers from the environment to provide a reference against which the other can be calibrated.
The existence of multiple modes of thickness shear vibration in quartz has been know for some time. U.S. Pat. No. 4,419,600 proposes a stress compensated dual mode transducer. U.S. Pat. No. 4,547,691 and U.S. Pat. No. 5,394,345 describe dual-mode transducers, an example of which is shown in FIG. 1. The resonators in such transducers have two vibrational modes at different frequencies, known as C mode and B mode. C mode is responsive to both pressure and temperature variation whereas the B mode is primarily responsive to temperature, the effect of pressure being relatively small. The structure of the dual-mode transducer again has a unitary resonator and housing structure 10. However, in this case, the resonator 25 lies in an axial plane of the cylindrical housing 12, the ends 32, 33 of the resonator 25 being unconnected to the housing 12. Again, electrodes are located on opposite faces of the resonator 25 to excite the vibrational behaviour.
Pressure and temperature transducers such as these find uses in borehole measurement tools such as are used in oil or gas wells. One example is the MDT Modular Formation Dynamics Tester of Schlumberger.
One characteristic of oil and gas wells is that often relatively high temperatures and pressures are encountered. Transducers of the type described above can operate in many of these environments but ultimately degrade in performance or generally fail completely once a certain pressure or temperature is exceeded. Since this is generally terminal, the transducer in question must be replaced completely. The common failure mode of such transducers is known as "twinning" and occurs when certain pressure and temperature limits are exceeded. The failure is due to irreversible changes in the crystal structure of the quartz resonator. The temperature and/or pressure at which these changes take place are dependent on the particular design of the transducer in question.
It is an object of the present invention to provide a transducer which can be made to operate reliably in high temperatures and pressures.